The present invention generally relates to a method for forming a dielectric thin film.
Currently, a piezoelectric element using a ferroelectric material (such as, lead zirconate titanate (Pb (Zr, Ti)O3, PZT)) is applied to MEMS (micro electromechanical systems) technique (such as, an inkjet head and an acceleration sensor).
FIG. 4 is a graph showing piezoelectric characteristics of a PZT thin film having a (100)/(001) orientation and a PZT thin film having a (111) orientation. The PZT thin film with (100)/(001) orientation is known to show a piezoelectric characteristic greater than the piezoelectric characteristic of the PZT thin film with the (111) orientation.
A description will be given of a conventional method for forming a dielectric thin film.
When a piezoelectric element is formed, as the substrate to be film formed, a Si substrate having a thermally-oxidized film on which a Ti thin film as an adhesive layer and a noble metal thin film as a lower electrode layer preliminarily laminated in this order, is used. The noble metal thin film is a Pt or Ir thin film preferentially oriented to a (111) plane.
FIG. 5 shows a temperature change of a heat generating member which heats the substrate.
The heat generating member is heated up to 640 degrees Celsius inside the evacuated vacuum chamber and held so as to set the substrate at an appropriate film forming temperature for forming a PZT thin film.
Sputtering gas is introduced into the vacuum chamber and an AC voltage is applied to the target; the introduced sputtering gas is ionized; and plasma is formed. Ions in the plasma sputter a surface of the target, and PZT particles are discharged from the target.
A part of the PZT particles discharged from the target enters a surface of the heated substrate; and thus, a PZT thin film is formed on the noble metal thin film of the substrate.
After the PZT thin film having a predetermined film thickness is formed, the application of the voltage to the target is stopped and the introduction of the sputtering gas is halted. A temperature of the heat generating member is lowered and held at 400 degrees Celsius so as to cool the substrate.
FIG. 6 shows X-ray diffraction patterns of three portions of the center portion (Center), the edge portion (Edge), and the middle portion (Middle) between the center portion and the edge portion in a PZT thin film formed on a Pt thin film using the conventional method for forming the dielectric thin film. It is understood that the PZT thin film to be formed is preferentially oriented in a (111) direction.
In other words, in the conventional method for forming the dielectric thin film, there has been a problem in that it is difficult to form a PZT thin film having a (100)/(001) orientation; see, for example, JPA No. 2007-327106, JPA No. 2010-084180 and JPA No. 2003-081694.